To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post LTE system’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like.
In the 5G system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
With rapid development of information industry, especially increasing requirements from mobile Internet and Internet of things (IoT), mobile communication techniques are facing unprecedented challenges. According to International Telecommunication Union (ITU) report ITU-R M.[IMT.BEYOND2020.TRAFFIC], it can be predicted that as of 2020, mobile service amount will increase 1000 times compared with 2010 (4G era), and the connected user devices will exceed 17 billion. With involvement of IoT devices into the mobile communication networks, the number of connected user devices may be more astonishing. Under the unprecedented challenges, communication industry and the academia have started intensive researches in fifth generation mobile communication techniques (5G) facing 2020. At present, architecture and global objective of future 5G have been discussed in the ITU report ITU-R M.[IMT.VISION], which provides detailed description to requirement prospect, application scenarios and various important performances of 5G. With respect to new requirement of 5G, the ITU report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] provides information related to technology trends of 5G, aims to solve dramatic problems such as system throughput, user experience consistency, extendibility, supporting IoT, tendency, efficient, cost, network flexibility, supporting of new services and flexible spectrum utilization.
Duplex mode in wireless communications refers to a processing manner of uplink and downlink bidirectional data communications and forms an important basis for air-interface design of the wireless communications, which is no exception in the research of 5G. At present, Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are two main duplex modes and have been widely used in broadcast audio and video fields and civil communication systems, e.g., Long Term Evolution (LTE) system corresponding to the Evolved Universal Terrestrial Radio Access (E-UTRA) protocol defined by 3rd Generation Partnership Project (3GPP), IEEE 802.11a/g Wireless Local Area (WLAN), etc.
In the FDD mode, uplink and downlink communications use paired frequency resources having a certain duplex spacing. However, in the TDD mode, uplink and downlink share the same frequency resources, and uplink communication and downlink communication are implemented via different time resources. Different duplex modes result in different physical layer designs for air interface such as frame structure. Take LTE as an example, two kinds of frame structures are defined in LTE respectively applicable for the FDD mode and the TDD mode.
The FDD frame structure is as shown in FIG. 1. Each radio frame is of 10 ms length, consists of ten 1 ms subframes. Each subframe consists of two 0.5 ms slots. Uplink communication and downlink communication are implemented using different frequency resources.
The TDD frame structure is as shown in FIG. 2. Similar as the FDD frame structure, each radio frame is of 10 ms length, consists of ten 1 ms subframes. The difference relies in that, the uplink communication and downlink communication in the TDD mode share the same frequency resources and are differentiated through time resources. For example, in the configuration as shown in FIG. 2, subframes 0, 5 are used for downlink communication, and subframes 2, 3, 4, 7, 8 and 9 are used for uplink communication. In order to ensure that the downlink communication does not affect the uplink communication, a special subframe is introduced in the TDD frame structure, i.e., the subframes 1 and 6 as shown in FIG. 2. The special subframe consists of a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Time Slot (UpPTS). In the TDD frame structure, subframes 1, 5 and the DwPTS are always used for downlink transmission, whereas UpPTS and its subsequent subframes are always used for uplink transmission. The GP is a guard period between the downlink communication and the uplink communication, so as to ensure that the uplink data communication is not affected by the downlink communication. The LTE TDD mode may be configured flexibly, so as to support uplink/downlink asymmetric services. Table 1 shows various configurations of the LTE TDD mode, wherein D denotes that the subframe is used for downlink communication, U denotes that the subframe is used for uplink communication, and S denotes the special subframe.
TABLE 1uplink-downlink configurations of LTE TDD modeDownlink-Uplink-uplinkdownlinkswitchingSubframe indexconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The above two duplex modes each have their advantages and disadvantages. In particular, the FDD mode requires paired frequency bands to implement uplink and downlink data communications, and the paired uplink and downlink frequency bands need a certain duplex spacing. In the case that 5G is developed towards high frequency and wide bandwidth, it may result in spectrum fragments from the perspective of spectrum division and thus is not good for spectrum management. The TDD mode uses the same spectrum for the uplink and downlink data communications. Therefore, the TDD mode has advantages in terms of spectrum utilization flexibility. It can support more asymmetric services and have higher spectrum utilization ratio. As to the FDD, since the spectrum is paired, the uplink and downlink resources are always available. Thus, the scheduling and the uplink control signaling fed back by the terminal are relatively in time, e.g., Acknowledge/Negative-Acknowledge (ACK/NACK) of Hybrid Automatic Retransmission reQuest (HARQ) and Channel State Information (CSI). As such, feedback delay of the air interface may be reduced, and the scheduling efficiency is increased. However, as to the TDD, the different uplink-downlink configurations lead to complex design. In addition, the TDD mode has the advantages of uplink/downlink channel reciprocity, which may greatly simplify the obtaining of the CSI.
Large-scale MIMO technique may be adopted in 5G to further increase the spectrum efficiency. The base station is equipped with a large amount of antennas. Thus, in the FDD mode, a large amount of resources may be required for downlink physical channel training and feedback of channel state information. However, the overhead of training and feedback may be greatly decreased utilizing the channel reciprocity under the TDD mode. Therefore, the TDD mode is more attractive for the large-scale MIMO technique. But 5G also has the requirement of low latency, and needs to further shorten the air interface transmission time interval (TTI) and control signaling, which makes the design of the TDD mode more complex.
It can be seen from the above analysis that, the FDD mode and the TDD mode respectively have their advantages and disadvantages. Facing the applications of rich application scenarios and new frequency band in 5G, it is necessary to design a new duplex mode, to combine the advantages of the FDD mode and the TDD mode, so as to ensure 5G spectrum utilization ratio and network performance better.